If you have many websites installed then your account usually have quite large amount of databases as well. So when you are trying to open phpMyAdmin via cPanel it loads quite slow.

Here's an official answer from cPanel support team which explains why it's loading so slow:

We have an internal case labeled CPANEL-12760 regarding this issue. This issue is related to the performance of information_schema in MySQL (and MariaDB and Percona) when it is accessed by a non-root user.The slowness is because phpMyAdmin scans information_schema to list all DBs. As root, it just does a select all with no constraints. For a user, it does select all WHERE = user, so it is scanning all of the dbs, then filtering. In addition to this, information_schema is not indexed. This is difficult for our developers to fix directly because:- This is the required schema for phpMyAdmin to work with MySQL and it is not something we can change or make more efficient.- We're not able to add indexing to Information_schema because it is Read-Only.- phpMyAdmin has submitted requests to MySQL to solve this problem and there has been no indication of solving.These bug reports describe the issue but are "closed" and they do not appear to have resolved the problem:https://bugs.mysql.com/bug.php?id=19588https://github.com/phpmyadmin/phpmyadmin/issues/10633https://github.com/phpmyadmin/phpmyadmin/issues/10638Our developers are investigating all possibilities to resolve this problem, even if we can only ask the MySQL developers to do so. We appreciate your patience and understanding as our developers continue to look into a possible solution to this upstream phpMyAdmin issue. Please do let us know if you have any additional questions or concerns.

Unfortunately we have not yet found a reliable way to speed it up for our customers, but we are looking into it actively.